The present invention relates to optical amplification apparatus for use in optical transmission systems and so on.
As a reduction in cost has been demanded for optical transmission systems, a wavelength-multiplex optical transmission has been taken into consideration for transmitting signal light at one or more mutually different wavelengths on a single transmission fiber. It is also thought that an amplifier suitable for use in such wavelength-multiplex optical transmission is an optical amplification apparatus which has a wide amplification wave band and is capable of achieving the amplification with less noise.
It is known, however, that a rare metal added optical fiber and a semiconductor optical amplifier constituting the above-mentioned optical amplification apparatus have a gain dependency so that optical outputs and gains at respective wavelengths present deviations due to the difference in wavelengths after amplification. For this reason, the optical power at different wavelengths after transmission involves a deviation due to the difference in wavelengths. Particularly, if a number of optical amplifiers are used to relay signal light at multiple stages, the deviation of optical power between different wavelengths, generated at respective relay stages, are accumulated as the signal light is relayed from one stage to next, thus increasing the deviation of optical power between the different wavelengths.
In the wavelength-multiplex optical transmission, since the wavelength signal having the lowest power of all multiplexed wavelengths must be regarded as a lower limit value of received power after transmission, a maximum transmission distance in the wavelength-multiplex transmission is limited by the wavelength signal having the lowest power. Thus, it is of great importance to reduce the deviation of power between different wavelengths in the output of an optical amplification apparatus, in order to extend a maximum relay transmission distance.
To solve this problem, an article titled xe2x80x9cCollective Smoothing of Multiple Wavelength Amplification Characteristics of Fiber Optic Amplifier Using Fiber Amplification Ratio Controlxe2x80x9d Technical Reports of the Institute of Electronics, Information and Communications OCS94-66, OPE94-88 (1944-11) has proposed the following technique.
FIG. 1 illustrates the configuration of an optical amplification apparatus according to the technique disclosed in the article. Referring specifically to FIG. 1, the optical amplification apparatus includes an erbium-added optical fiber 51, an optical isolator 52, a light combiner 53, an excitation light source 54, an optical attenuator 55, an optical coupler 56 for splitting the output of the optical attenuator 55, and a light detector 57 for detecting split light.
In the disclosed technique, the illustrated optical amplification apparatus is controlled by an auto fiber gain controller (AFGC) such that a fiber gain remains at 12 dB, thereby minimizing a deviation of gain between respective wavelengths. In addition, an auto power controller (APC) implemented by the optical attenuator 55 is used to prevent a change in relay amplification ratio from affecting the gain spectrum.
It has been reported that, according to theoretical calculations, the optical amplification apparatus presented a minimum gain deviation between respective wavelengths, which is 0.12 dB or less, when the erbium-added optical fiber 50 had a length of 11 meters, assuming that a deviation of gain between the respective wavelengths of input light was 0 dB. It has been also reported that after the optical amplification apparatus has been used to relay light having four different wavelengths multiplexed thereon 60 times, a gain deviation was 1.5 dB or less.
Optical losses during transmission may vary from one case to another due to a difference in fiber loss within each relayed area, a difference in optical power between adjacent wavelengths, and so on. Additionally, in an actual use, relayed distances and fiber losses in respective areas are not always constant. It is therefore difficult to predict a deviation of gain between respective wavelengths and optical power at the respective wavelengths in an actual use. Therefore, the optical amplification apparatus illustrated in FIG. 1 has a problem in an actual use that if an input level changes or if a deviation of gain occurs between input wavelengths, the optical amplification apparatus cannot reduce a deviation of gain between output wavelengths to 0 dB.
Also, when the optical amplification apparatus illustrated in FIG. 1 is used, if an independent fluctuation in output power of signal light at a certain wavelength caused by an external factor, for example, is to be suppressed, stable output power of signal light at the remaining wavelengths is also suppressed simultaneously, thus adversely affecting the stability of the output power of the signal light at the different wavelengths.
Further, since the optical amplification apparatus illustrated in FIG. 1 is dependent on the gain thereof for establishing an optimal condition for eliminating the deviation of gain between wavelengths, it cannot freely set outputs of signal lights. More specifically, since a relayed distance is limited by the optical amplification apparatus the freedom in designing the architecture of a transmission system is restricted. The optical amplification apparatus illustrated in FIG. 1 additionally has a problem that it must be optimized to eliminate a deviation of gain between wavelengths in each relay area.
It is a principal object of the present invention to provide an optical amplification apparatus which is capable of arbitrarily adjusting optical output power at respective wavelengths of wavelength-multiplexed signal light and a deviation in optical power between the respective wavelengths.
It is another object of the present invention to provide an optical amplification apparatus which uniformly increases or decreases input power of signal light at respective wavelengths inputted thereto and an amplification ratio of the optical amplification apparatus to thereby generate an output which is dependent on an increase or a decrease of the input power.
To achieve the above objects, the present invention positions an optical power adjusting means which receives inputted light having signal light at a plurality of different wavelengths multiplexed thereon for amplifying or attenuating light at at least one wavelength included in the received light independently of the remaining light at different wavelengths before or after an optical amplifying means for amplifying the light having the signal light at the plurality of different wavelengths multiplexed thereon. Further, a control means is provided for controlling the gain of amplification or attenuation performed by the optical power adjusting means and the gain of amplification performed by the optical amplifier, respectively.
The present invention will be explained below in connection with an example in which the optical power adjusting means is provided before or after the optical amplifying means.
A rare earth added optical fiber or a semiconductor amplifier commonly used as the optical amplifying means has output power dependent on input power on condition that excitation power is constant. This also applies when multiplexed light having light at wavelengths xcex1, xcex2, xcex3, . . . , xcexN multiplexed thereon is simultaneously amplified. Therefore, if the optical amplifying means is driven to increase or decrease the input power of the light at the respective wavelengths, it is possible to produce output power dependent on the increased or decreased input power.
Thus, in the present invention, the optical power adjusting means is positioned before the optical amplifying means. The optical power adjusting means receives light having multiplexed thereon light at a plurality of wavelengths and amplifies or attenuates light at at least one wavelength included in the received light independently of other light at different wavelengths from the wavelength of the light to be amplified or attenuated. The optical power adjusting means adjusts a deviation of optical power between the wavelengths of the light at the respective wavelengths inputted to the optical amplifying means, and thereafter the optical amplifying means simultaneously amplifies the light having the light at the respective wavelengths multiplexed thereon, thereby adjusting the power of the light at the respective wavelengths and the deviation of optical power between the wavelengths to desired values.